The present invention relates to a cooling system for cooling a turbine of a gas turbine engine.
The high pressure turbine components of an aero gas turbine engine are located in the hottest part of the engine. At around 1600° C., the temperature of the gas stream is greater than the melting temperature of the nickel-based alloys from which the high pressure turbine nozzle guide vanes and rotor blades are typically cast.
It is usual, therefore, to cool nozzle guide vanes and rotor blades internally using cooling air taken from the exit of the high pressure compressor. The cooling air, which bypasses the combustor, may nonetheless be at a temperature of over 700° C. Nozzle guide vanes and rotor blades can also be cooled externally by passing the cooling air through passages connecting the interior to the exterior of the respective component so that the cooling air forms an air film around the component to protect it from the hot gas stream. In order to form the air film, the cooling air in the component must have a positive pressure relative to the exterior gas stream pressure.
As the nozzle guide vanes are static components, they can be supplied with cooling air by relatively straightforward ducting arrangements. Cooling air is typically supplied to the rotor blades after passing through a preswirler which imparts a swirling motion to the cooling air to match the rotation of the rotor disc. The swirling cooling air impinges on the radially outer portion of the rotor disc to cool the disc, and enters internal cavities in the rotor blades via entrance holes in the root regions of the blades.
Although necessary to protect turbine components from the hot gas stream, the extraction of cooling air from the compressor has an adverse effect on engine efficiency. Therefore, the cooling air should be used as effectively as possible.
U.S. Pat. No. 4,254,618 proposes bleeding cooling air from the compressor discharge of a turbofan engine, and routing the cooling air to a heat exchanger located in a diffuser section of the fan duct. The cooled cooling air is then routed through compressor rear frame struts to an expander nozzle and thence to the turbine.